An aim of the present invention is:                plates, having two approximately parallel main sides, of a glass-ceramic or a glass displaying a certain transparency, with a colored coating fixed to at least one part of the surface of one of said two main sides;        heating plates, particularly cook-tops, consisting of such plates or obtainable from such plates. These heating plates are intended to be fitted above heating elements, their side bearing said coating facing said heating elements; and        the preparation of such plates.        
The plates of the invention—which are transparent; in any case, sufficiently transparent so that their colored coating be perceptible through their thickness—are characterized by the specific nature of their colored coating.
The heating plates of the invention are particularly efficient, notably with reference to the demanding specifications specified below.
Cook-tops have already been described according to prior art, mainly of glass-ceramic and of toughened glass, with a colored coating on their “lower” side (or reverse side or under side), i.e. the one which is intended to be deposited facing the heating elements.
In the patent application EP-A-277 075, the presence of decorative or marking elements is very generally provided on the reverse side of a glass-ceramic plate which is intended for cooking by induction heating. No precision is provided on the decorative or marking elements in question.
In the patent application EP-A-1 267 593, transparent glass-ceramic plates are described with colored coatings underneath. Said plates are suitable for induction heating, even combined heating: induction heating and radiant heating. The colored coatings in question are enamels with metallic pigments, which are themselves optionally covered with an organic coating (such as a polyimide, polyamide, fluorinated, silicone resin) on the “hot” surfaces.
In the patent application EP-A-861 014, the presence of a paint layer, which is resistant to high temperature, preferably to temperatures of greater than 650° C., is provided on the whole of the side opposite to the one which is intended to receive cooking utensils. Glass-ceramic heating plates are in question, but no precision is provided on the exact nature of the paints mentioned.
Patent applications U.S. Pat. No. 2002/0 084 263 and U.S. Pat. No. 2003/0 019 864 describe glass-ceramic and toughened glass substrates, which are transparent, for use as cooking surface with radiant heating. Said substrates have (U.S. Pat. No. 2002/0 084 263) or do not have (U.S. Pat. No. 2003/0 019 864) a coating of enamel type on their upper side. On their lower side, turned towards the radiant heating elements, the substrates have colored coatings, which are permeable to infra-red, of different natures (notably sol-gel layers with pigments), and which are adapted to the zone to be decorated, more specifically to the temperatures to which they are led to undergo during use.
The application FR-A-2 838 429 describes glass-ceramic plates which are intended notably for covering heating elements and which are provided over at least one side with at least one enamel solid and/or with at least one paint layer. The paint layer is advantageously present on the lower side of the plate, with the exception if need be of the heating zones. The paint used advantageously has a degradation temperature of greater than 350° C., generally between 350 and 550° C. Preferably, this is a paint based on silicone resin(s) and particularly preferably, the paint comprises one or more silicone alkyd resins (one or more silicone resins modified by the incorporation of alkyd resin(s)). This paint can also comprise pigments, e.g. TiO2. The means of heating provided is essentially induction heating.
In such a context, the inventors have conceived novel plates, of glass-ceramic and of glass, with a colored coating fixed to their lower side, which are particularly efficient in terms of:                stability, in use, of the color of said colored coating;        stability, in use, of the cohesion of said coating and of its attachment to the lower side of the plate;        adaptability of said coating to induction heating (with use temperatures of up to 350° C., eventually temperature peaks up to 500° C.) and to the heating with halogen or radiant electric heating elements (with use temperatures of 650 to 700° C.); and        mechanical properties: the mechanical properties of said plates are not affected by the presence of said coating. On the contrary, even an improvement of said properties is observed.        
The technical problem tackled was that of the stability of the color of such a coating with temperature. The coating, which is the key of the invention, proposed with reference to this technical problem has, in an entirely surprising way, revealed to be efficient with reference to all the points of the specifications above, this being despite an evolution of its nature from organic to inorganic. This affirmation is understood better upon consideration of the following.